There's an article on rebuilding the Ensign interior on the ECA website:
Library -> Boat Maintenance and Repairs -> Rebuilding the Ensign Interior
You must be logged in to the website to view the full article. Log in first, then click:
The article is well beyond simply rebuilding the sole/mast step area, but it does show putting flotation in the bow area (which was accomplished with 2" closed foam sheets purchased at Home Depot with any spaces filled with Great Stuff aerosol foam.) A multitool is indispensable in the removal of the old foam and the shaping of new, replacement pieces.
Regardless of how the boat is used (racing/cruising, etc), replacing the sole itself is pretty important because it resists the compression of the mast and the rig tension.
The Ensign will not hold rig tension if the sole is soft. Compression on the mast will just press the mast base / sole further downward into the bilge at some maximum load. In the worst case scenario of soft sole / high wind-rig tension-compression, mast failure can occur, so this common ailment is something to fix properly.
If you need to wait until next winter's repair season, you can use a temporary fix of 1/4"+ thick, aluminum plate, cut into a trapezoidal shape (narrow at the forward edge, wide at the aft edge, with sides angling away from each other from front to back) to fit the sole area underneath the mast step, between the settees. The plate will grab a lot of sole surface area and help transfer the load from the sole middle to the edges, where the hull is almost directly underneath.
When you do the project, I recommend replacing the sole with 3/4" G10 plate, and replacing the four, underlying, (probably now ineffective) athwartship mast / sole supports with 3/4" Coosa board, filleted on all edges of contact with a WEST System - colloidal silica mix, tabbed in with fiberglass mat.
This is a one-and-done-forever solution with very little possibility of future degradation or rot.
BB
You can always call to discuss.
281.468.6909